WiFi through walls - Directional vs Omni Directional

Discussion in 'Wireless Internet' started by Dennis, Jun 7, 2008.

  1. Dennis

    Dennis Guest

    Do directional antennas only do well in line-of-sight situations?

    I need to go through about 4 or 5 walls to get to my router. Would I
    do better replacing my NIC's 2 dBi omni-directional antenna with a 7
    dBi, or would I have better luck with a 12 dBi directional antenna
    aimed at the router (through a bunch of walls)?
     
    Dennis, Jun 7, 2008
    #1
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  2. Depends how the signal is getting through the wall!

    Broadly speaking you can have two types of directional antenna: one which is
    still omnidirectional in the horizontal plane with increased gain, at the
    expense of the vertical plane which is reduced, i.e. isn't very sensitive to
    signals coming in from above it.

    The other is increased sensitivity in both horizontal and vertical planes (a
    bit like the inverse of a searchlight).

    So if you can "point" towards where most of the signal is entering the room
    you will get an improvement.

    HTH Regards, Charles
     
    Charles Turner, Jun 7, 2008
    #2
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  3. Dennis

    tg Guest

    I had a very similar problem recently. wifi hates walls, especially 4 or 5 of them. If you
    have no luck connecting with your 7db antenna try network-over-mains.
    I'm in the UK and I got these:
    http://www.wirelesspro.co.uk/product/budget_85_mbps_homeplug_kit/
    If you're outside the UK there must be an equivalent for your country.
    They great, and really are plug and play. There's no set up, no configuring, just plug
    them in connect the ethernets at each end and you're away.
     
    tg, Jun 7, 2008
    #3
  4. No. Directional antennas can be used for non-line-o-site situations.
    Give up now, while you're still sane. Unless the walls are made of
    paper, it's not going to work. Most walls are good for considerable
    losses. If there's aluminum foil backed insulation in the walls,
    nothing gets through. Fairly lossy with poured concrete and chicken
    wire backed stucco or lath and plaster.
    Neither. Drill a hole under the trim and run CAT5. When you move
    out, patch the holes with spackle, and replace the trim. Nobody will
    notice. Also CAT5e in flat cables which does nicely through cracks,
    under carpets, the under doors. If impossible, think about power line
    networking.
     
    Jeff Liebermann, Jun 7, 2008
    #4
  5. Dennis

    Dennis Guest



    I should have been more clear. This is not mission impossible. Even
    with my 2 dBi antenna I am receiving a signal, at about 15% strength.
    Perhaps do to multipath, open doors and open windows, but I am getting
    something.
    Enough to surf the web, slowly...

    See this YouTube video:

    That's kinna what got me thinking about it. He's a directional setup
    to go through walls, he picks up 200+ access points at one point. Very
    interesting. Granted its a *completely* different situation, but I
    dont think his results would be nearly as good with any kind of omni-
    directional setup.
     
    Dennis, Jun 8, 2008
    #5
  6. What type of walls? Usually, one wall is no problem. Two walls are
    possible. Three or more walls are usually a problem. You're correct
    and probably are dealing with multipath and reflections. The problem
    with such a path is that it's not reliable. You're going to be
    adjusting your antenna every time someone moves something. It's
    really not reliable, no matter how strong the signal.

    There's also some question as to where to point it. Directly towards
    the access point is the obvious start, but if you're dealing with
    reflections, other less obvious directions, might work better. If
    this is the case, they won't stay working as the reflections tend to
    move around. You will probably be constantly adjusting the antenna.
    What do you have for equipment? USB, PCI, laptop, or what? If it's a
    USB or laptop, almost any manner of external antenna is better than
    what comes stock. The gain of the USB devices are typically about
    -2dBi. Yeah, it's negative as in a net loss of power. Literally any
    external antenna is better than that.

    Same with some laptops, but for a different reason. The thin coax
    cable going between the wi-fi card in the base, and the diversity
    antennas near the top of the LCD, is rather lossy. There are also
    some laptops that stupidly install the antenna in the hinge, which is
    really bad for performance:
    <http://802.11junk.com/jeffl/antennas/laptops/slides/compaq-2120us.html>

    Whatcha got to work with in the way of hardware?
    The guy is probably half way up the high rise apartments in the photo
    and is looking out over a very large area, with plenty more wireless
    devices. He could be using just the USB dongle, and probably get a
    substantial number of access points. It would have been interesting
    if he had run the same test with just the USB dongle fpr comparison.

    I did the numbers for such a contrivance in the past.
    <http://groups.google.com/group/alt.internet.wireless/msg/bae6f7c6d35160d0>
    19.5dBi maximum gain for a 0.6 meter dish. That's probably high
    considering that the illumination angle of the coffee can feed, does
    not match that of the dish, and the illumination pattern of the USB
    device does not match that of the coffee can. Still, my guess(tm) is
    that a gain of 15dBi can be achieved with such a setup.
    Do you really want that mess indoors? If not, look into panel
    antennas, which are less aesthetically disgusting.

    Anyway, it's worth trying. The setup in the YouTube video is better
    than most construction articles. This one is also tolerable:
    <http://www.weijand.nl/wifi/>
    However, methinks you should try a different flavor of reflector
    before bulding something like the YouTube video. See:
    <http://www.freeantennas.com>
    for various simple reflectors (that work).
     
    Jeff Liebermann, Jun 8, 2008
    #6
  7. Dennis

    Dennis Guest

    Lots of good info there Jeff, thanks its appreciated.

    For fun I went out and grabbed a $20 omni antenna, it claims to be 9
    dBi... I figured for $20 it was worth a shot... Until I realized I'd
    also need an SMC cable.. the new antenna needs to be raised away from
    the case of the computer like the old 2 dBi one was.... surfing the
    net I'm now realizing the cable is going to cost more then the antenna
    did :(

    Which brings me to my next question: How much loss is there per foot
    of cable (50 ohm) I use? Is it significant enough for me to want to
    get the shortest cable possible, or can I give myself some leeway and
    get a 20-30 foot cable?
     
    Dennis, Jun 10, 2008
    #7
  8. Dennis

    ps56k Guest

    at the Wifi freqs - 2.4Ghz microwave - there is a LOT of loss per foot.
    Here's a chart for common LMR type cables ...
    http://www.universalweb.com/CustomCablesUSA/PDF/RF-LMR-Cables-Specs_CustomCablesUSA.pdf
     
    ps56k, Jun 10, 2008
    #8
  9. Dennis

    seaweedsl Guest


    Short answer is: LMR 400 loses .25 db per meter or 6.6 for 100 feet.
    30 foot cable is only 2 db. Hawking has a 30 ft 400 that can be
    found for $30. Will need a pigtail-adapter, too.

    You can use this to determine loss for a given cable:
    http://www.timesmicrowave.com/cgi-bin/calculate.pl

    Enter 2400 Mhz for frequency if you using B or G.
    You could use your existing cable for testing and just get an adapter
    to start?
     
    seaweedsl, Jun 10, 2008
    #9
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